The present invention is directed to charge generation layers which comprise a charge generation compound such as titanyl phthalocyanines. The invention is also directed to photoconductors including such charge generation layers.
In electrophotography, a latent image is created on the surface of an imaging member such as a photoconducting material by first uniformly charging the surface and then selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner. Electrophotographic photoconductors may be a single layer or a laminate formed from two or more layers (multi-layer type and configuration).
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to use of a charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than a hole transport material, the charge placed on a photoconductor surface will be opposite that described herein.
When the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer comprises the charge generation compound or molecule alone and/or in combination with a binder. A charge transport layer typically comprises a polymeric binder containing the charge transport compound or molecule. The charge generation compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate electron hole pairs therein as a result of absorbing such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of a negatively charged photoconductor. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No. 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
Typically, the charge generation layer comprises a charge generating pigment or dye (phthalocyanines, azo compounds, squaraines, etc.), with or without a polymeric binder. Since the pigment or dye in the charge generation layer typically does not have the capability of binding or adhering effectively to a metal substrate, the polymer binder is usually inert to the electrophotographic process, but forms a stable dispersion with the pigment/dye and has good adhesive properties to the metal substrate. The electrical sensitivity associated with the charge generation layer can be affected by the nature of polymeric binder used. The polymeric binder, while forming a good dispersion with the pigment should also adhere to the metal substrate.
Improvement in print quality is always desirable, especially in the case of color printers since they exhibit an outstanding range of graphic capabilities. Such a range is a function of gray scale capabilities, and gray scale is obtained by printing intermixed color and background in patterns of very minute elements. This invention achieves improved gray scale by controlling photoconductor sensitivity so as to have more consistent response.
Such response is obtained in accordance with this invention by employing both type I titanyl phthalocyanine and type IV titanyl phthalocyanine. Surprisingly, these materials function by combining their level of photosensitivity so that the desired photosensitivity can be reliably reproduced. Preferably, the type I titanyl phthalocyanine is premilled before milling the mixture.